In order to assist a steering force of an automobile or the like, in recent years, a large number of vehicles are provided with so-called power steering devices. As the power steering devices of this type, recently, the number of vehicles to which electric type power steering devices (so-called electric power steering device, hereinafter appropriately abbreviated as “EPS”) are mounted are increasing from a viewpoint of a reduction in engine load, a reduction in weight, or the like. As a power source of the EPS as described above, motors with brushes have been frequently used hitherto. In recent years, however, the brushless motors have been increasingly used because the brushless motors are excellent in maintenance property, have small size, and are capable of generating high torque.
In such ESP motor as described above, attention is focused on an operation noise reduction for improvement of driver's operation feeling. In general, the torque variation (torque ripple) and the operation noise are closely related with each other, and torque ripple reductions realized by designing an energizing method have been variously studied for the EPS motor, particularly, the brushless motor. For example, it is well known that a sine wave drive that enables smooth energization is conducted instead of a square wave drive as a measure against the operation noise. However, in the case of the sine wave drive, an induced voltage waveform on the motor side needs to be converted into a sine wave configuration in order to reduce the torque ripple. For that reason, in the sine wave drive motor, the induced voltage waveform is converted into the sine wave waveform by skew or eccentricity of a magnet.
However, when the rotor is subjected to skew, a first-order component of the induced voltage is also reduced. For that reason, in the conventional sine wave drive motor, an output of the motor must be somewhat sacrificed under the circumstances. Therefore, in order to improve a reduction in motor output as described above, there has been developed a technique in which a harmonic is included in the induced voltage, and a current that cancels the harmonic is allowed to flow, thereby reducing a skew angle to reduce an output loss. Further, for example, Patent Document 1 proposes a technique in which the induced voltage of each phase is adjusted to a trapezoidal wave on which an odd-order harmonic is superimposed to improve the torque while suppressing a torque ripple. In the technique, induced voltage adjusting means superimposes a third-order harmonic on the induced voltage of a sine wave configuration being a fundamental wave to adjust the induced voltage of each phase into a substantially trapezoidal waveform configuration flatly spread without a rapid change. As a result, a configuration of a portion surrounded by a characteristic curve of each phase induced voltage with respect to a rotor rotation angle and an abscissa axis (rotation angle) is formed into a configuration more flatly spread than that in the case of the characteristic curve of only a fundamental wave, and motor generation torque is increased by that amount.    Patent Document 1: JP 2006-174692 A    Patent Document 2: JP 2004-274693 A
However, when the harmonic is superimposed on the induced voltage, and the induced voltage of each phase is formed into the trapezoidal waveform as described above, it is difficult to suppress a peak current value because the phase current of each phase is formed into the sine waveform. For that reason, there arises such a problem that a current value that can be made to flow in the brushless motor is restricted, and the motor output is suppressed by that amount.
An object of the present invention is to provide a brushless motor and a method of controlling a brushless motor capable of realizing higher torque and higher rotation compared with the drive system in which the induced voltage of each phase is formed into the trapezoidal waveform, while suppressing the peak current value.